1. Field of the Disclosure
The present disclosure generally relates to methods and apparatus for monitoring subsurface formations. More particularly, the present disclosure relates to methods and apparatus for sensing and monitoring acoustic activity, and specifically microseismic activity, in subsurface formations.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
Oil reservoirs are becoming more and more depleted and require proactive methods in order to maximize the hydrocarbon or oil recovery. One of those proactive methods includes pumping operations conducted in oil and gas wells to increase production and recovery rates of producer or injector wells. The pumping of fluid is required for a variety of operations including hydro-fracturing and acidizing, collectively referred to as “stimulation”. Stimulation is used to increase conductivity of a subterranean formation for recovery or production of hydrocarbons and to permit injection of fluids into subterranean formation. For instance, in injector wells, it is common to stimulate an injection zone by placing an acid to increase injectability rates; in the production of hydrocarbons from the earth, it is widespread to hydraulically fracture a producing formation to increase its permeability and subsequent production.
In a typical hydraulic fracturing operation, a fracturing fluid is injected under pressure into the formation through an injection well. This fluid may carry entrained particulate matter such as sand or gravel (known as proppant) that is deposited in the fracture as it is formed to hold open the fracture after the fracturing pressure is relaxed. Another similar application is one in which waste material is mixed into a slurry with the fracturing fluid and injected for disposal in the subterranean formation.
One important consideration for stimulating or fracturing for hydrocarbon production and waste disposal is that the fracture should be directed toward the most desirable part of the formation. The formation should not only be capable of being stimulated or hydraulically fractured, but containment barriers should also be bounding it. Unfortunately most formations do not have ideal characteristics for stimulation. Consequently, it is important to monitor the placement of the stimulation fluid as it is being pumped to insure that it does not extend beyond the intended zone.
One method of monitoring the location and size of a hydraulic fracture is called microseismic mapping. In this method, a plurality of acoustic sensors (e.g. geophones) are positioned in spaced (i.e., offset from the injection) wells, such as adjacent or nearby monitoring wells, and the sensors are used to record signals from micro-earthquakes (microseisms) caused by the stress induced by the fracture fluid pressure build up. The microseisms are localized and occur during the growth of fractures. The amplitude of the seismic or acoustical energy (compressional (“P”) waves and shear (“S”) waves) are generated with significant enough amplitude to be detected by acoustic sensors. Accordingly, by sensing and recording the P and S waves and their respective arrival times at each of the sensors, the acoustical signals can be processed in accordance with known seismic and/or earthquake monitoring methodology to determine the position of the microseisms. Hence the geometry of the fracture and its location may be inferred.
As noted above, this type of pumping operation is conducted in a near or adjacent monitoring well, such that the producer or injector well can be monitored from a distance without the monitoring equipment causing interference in the producer or injector well. However, monitoring wells are not always available or suitable for such monitoring (e.g., if located too far away from the injection well). An alternate approach would be to conduct the monitoring in the producer or injector well, which, as stated previously, introduces the issue of interference. Moreover, using the producer or injector well poses the problem of obtaining precise readings of microseismic data in a high noise environment, mostly resulting from the pumping and delivery of the fracture fluid.
Accordingly, the present disclosure is directed to a method and apparatus for integrating stimulation and monitoring of earth formation from inside the stimulated well.